1. Field of the Invention
The present invention relates to a polyethylene powder.
2. Description of the Related Art
Ultrahigh-molecular-weight polyethylene powders have a viscosity-average molecular weight (Mv) that reaches 1,000,000 to 9,000,000 and are excellent in abrasion resistance, impact resistance, self-lubricating properties, chemical resistance, low-temperature characteristics, dimensional stability, light weight, safety for foods, etc. Therefore, these polyethylene powders are molded by various molding methods such as melt drawing, injection molding, extrusion molding, and compression molding and employed in various uses such as for films, sheets, microporous membranes, fibers, foams, and pipes. Particularly, the ultrahigh-molecular-weight polyethylene powders are used as raw materials for microporous membranes for separators of secondary batteries typified by lead storage batteries and lithium ion batteries and as raw materials for high-strength fibers.
In general, these ultrahigh-molecular-weight polyethylene powders are highly viscous when melted. Such ultrahigh-molecular-weight polyethylene powders are difficult to process by injection molding or the like and are therefore often molded after being dissolved in a solvent.
Particularly, ultrahigh-molecular-weight polyethylene threads are obtained as high-strength fibers, for example, by suspending an ultrahigh-molecular-weight polyethylene powder in a solvent or swelling and dissolving the ultrahigh-molecular-weight polyethylene powder in a solvent under heating, and feeding this suspension or solution to a screw extruder, followed by extrusion from the spinneret of a die and drawing.
In this operation, the ultrahigh-molecular-weight polyethylene powder having low solubility in the solvent yields an undissolved powder residue, which becomes responsible for the clogging of a screen mesh in an extruder. Even if spinning can be achieved, unmelted components disadvantageously serve as a point of rupture so that end breakages occur during drawing. Particularly, in the case of threads, the exhibition of higher strength requires higher drawing. Thus, the undissolved powder residue causes disadvantageous end breakages during production and becomes a major problem.
In recent years, ultrahigh-molecular-weight polyethylene having a higher viscosity-average molecular weight has also been used for attaining the higher strength of threads or the lighter weights of products produced from the threads. Unfortunately, problems associated with the undissolved residue have occurred more frequently.
Some methods have been disclosed as methods for producing ultrahigh-molecular-weight polyethylene. For example, a method for producing ultrahigh-molecular-weight polyethylene having both of a narrow molecular weight distribution and a high bulk density by use of a mixed catalyst consisting of a titanium component and an organic aluminum compound has been reported (Patent Literature 1).
A method for producing ultrahigh-molecular-weight polyethylene that has a narrow molecular weight distribution and a high bulk density, has a narrow particle size distribution, is free from fine powders, and is excellent in the shape of particles by use of a metallocene-supported catalyst has also been reported (Patent Literature 2).
A method for producing ultrahigh-molecular-weight polyethylene having a wide molecular weight distribution and a low bulk density by use of a mixed catalyst consisting of a titanium component and an organic aluminum compound has been further reported (Patent Literature 3).